Rework the logic to not send double notifications in session_notification().

Rework the logic to not send double notifications in session_notification().

last_sent_errcode is now cleared late (when state changes to ESTABLISHED)
and so notifications sent in the OPEN phase would be dropped after the
first incident. Using the session state to know if sending a message is
possible seems more robust and is what session_stop() uses as well.
For now log the notification which are not sent as 'dropping' to see how
often this happens.

OK tb@